We evaluated the response of adult volunteers to a series of ts or cold adapted recombinants of influenza A virus. A/Victoria/75 ts-1(E) recombinants, with a 38 degrees C shut off temperature and ts lesions in RNA segments 1 and 5, retained some virulence for individuals who lacked immunity to the hemagglutinin and neuraminidase antigens of the virus. In contrast, a more temperature sensitive A/Victoria/75 ts-1A2 recombinant, with a 37 degrees C shut off temperature and ts lesions in RNA segments 1 and 2, was satisfactorily attenuated for doubly seronegative volunteers. Contrasting responses were observed with 2 ca recombinants; A/Scotland/74 ca retained some virulence, while A/Victoria/75 ca was satisfactorily attentuated. BIBLIOGRAPHIC REFERENCES: Richman, D.D., Murphy, B.R., Chanock, R.M., Gwaltney, J.M., Jr., Douglas, R.G., Betts, R.F., Blacklow, N.R., Rose, F.B., Parrino, T.A., Levine, M.M. and Caplan, E.S.: Temperature-sensitive mutants of influenza A virus. XII. Safety, antigenicity, transmissibility and efficacy of influenza A/Udorn/72-ts-1(E) recombinant viruses in human adults. J. Infect. Dis. 134: 585-594, 1976. Kim, H.W., Arrobio, J.O., Brandt, C.D., Parrott, R.H., Murphy, B.R., Richman, D.D., and Chanock, R.M.: Temperature-sensitive mutants of influenza A virus: Response of children to the influenza A/Hong Kong/68-ts-1-(E) (H3N2) and influenza A/Udorn/72-ts-1(E) (H3N2) candidate vaccine viruses and significance of immunity to neuraminidase antigen. Pediatr. Res. 10: 238-242, 1976.